Most antibody drugs are conventionally identified by screening for antibodies that bind to either a cell-surface receptor or its cognate ligand, and identifying antibodies that specifically block or stimulate the receptor signaling activity. Many antibody drugs block signaling pathways by binding to either the ligand or receptor, thereby eliminating the ability of the ligand to bind to and activate the receptor. Such blocking antibodies mediate their effect stoichiometrically by preventing the formation of receptor-ligand complex. Conversely, some antibody drugs bind to and activate signaling of a receptor. Such agonist antibodies may mediate their effects by mimicking the natural activity of the ligand and thus do not require the presence of ligand to activate signaling.